This invention relates generally to clutches. More particularly, it relates to a clutch for use in effecting locking engagement between a drive axle and a wheel of a four-wheel drive vehicle upon the application of power to the drive axle. The clutch automatically effects disengagement upon cessation of the application of power to the drive axle, together with a direction reversal.
Heretofore, various mechanisms have been used for engaging a drive axle with its associated wheels in a four-wheel drive vehicle. One such mechanism in the form of a wheel hub normally is disengaged to allow the wheel to rotate independently of the axle. This requires that the operator lock a pair of hubs manually to engage the drive axle and wheels, and to unlock them manually to disengage.
Another such mechanism provides an overrunning clutch which engages automatically when power is applied to the drive axle and when operation is in the drive mode. However, such an overrunning clutch inherently disengages upon operation in the coast mode. In other words, the overrunning clutch engages when the rotational speed of the axle tends to exceed the rotational speed of the wheel, but disengages when the rotational speed of the wheel tends to exceed that of the axle. Such an overrunning clutch generally provides some means by which the operator may override manually to insure locking engagement between the axle and wheel.
Yet another such mechanism provides a clutch which operates in response to the application of power to the drive axle by moving pins into slots so as to engage the axle with its associated wheel. Although a mechanism of this type will effect engagement in either the drive or coast mode of operation, there is the possibility that the pins could slip out of the slots during movement between the drive and coast modes, in which case the clutch would disengage and then re-engage automatically. At normal operating speeds such disengagement and re-engagement could cause severe shocks to the clutch components and, indeed, to the entire driveline. This would result in a dangerous and possibly destructive condition. Further, in a float condition wherein the axle is rotating but no torque is transferred between the axle and wheels, an inadvertent tendency for movement between the drive and coast modes could develop. This also could cause the clutch to disengage and then re-engage, thus establishing the same dangerous condition.
U.S. Application Ser. No. 799,793 filed May 23, 1977 is directed to an automatic locking clutch which overcomes the deficiencies of the prior mechanisms. As disclosed therein, the clutch automatically engages a drive axle and an associated wheel upon engagement of a four-wheel drive system, maintains engagement positively in the drive and coast modes of operation as well as during the transition between drive and coast, maintains engagement positively in the forward and reverse modes of operation as well as during the transition between forward and reverse, and disengages automatically upon a slight direction reversal when the four-wheel drive system is disengaged.
A deficiency of this clutch is that it includes a ring which develops frictional drag sufficiently high to effect clutch engagement. Thereafter, this high drag continues even though it is not required to maintain clutch engagement. Thus, in order to conserve power, reduce heat, ease the problems of material selection, etc., there was still a need for an automatic locking clutch of this type which would develop relatively high frictional drag initially to effect automatic clutch engagement, and would develop relatively low frictional drag thereafter.
U.S. Application Ser. No. 868,587 filed Jan. 11, 1978 is directed to an automatic locking clutch which meets this need. This clutch includes a mechanism for developing frictional drag to effect clutch engagement. The mechanism incorporates a wrapped spring characterized as having relatively high resistance to slippage in the spring-tightening or wrapping direction, and relatively low resistance to slippage in the spring-loosening or unwrapping direction. Upon initial rotation of the driving member, the spring is caused to tighten, thereby effecting clutch engagement. Thereafter, the spring is caused to loosen, thereby reducing frictional drag to a minimum.
One problem with such an arrangement is that there may be times when movement toward clutch engagement is blocked or otherwise restrained. In such an event, rotation of the driving member causes the wrapped spring to tighten and develop relatively high frictional drag until clutch engagement is completed. It had been thought that this condition would obtain for only one revolution or less. It is known now that this condition could obtain for a much longer time, resulting in the development of excessive heat and even possible seizure of the wrapped spring.
Another problem is that there may be times when spurious rotation of the driving member could initiate clutch engagement. This might occur, for example, when a vehicle is operating in two-wheel drive during cold weather. Viscous drag within an associated transfer case could cause spurious rotation of the driving member, which spurious rotation could be sufficient to initiate the engaging sequence.